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NEW QUESTION # 107
Refer to the code below:
01 x = 3.14;
02
03 function myFunction() {
04 'use strict';
05 y = x;
06 }
07
08 z = x;
09 myFunction();
Considering the implications of 'use strict' on line 04, which three statements describe the execution of the code?
Answer: A,B,C
Explanation:
:
Behavior of non-strict global code
The script does not begin with a 'use strict' directive at the top level, so the global code (outside any function) runs in non-strict (sloppy) mode.
Line 01: x = 3.14;
In non-strict mode, assigning to an undeclared identifier (no var, let, or const) creates an implicit global variable. So after line 01, x exists and equals 3.14.
Line 08: z = x;
This also runs in non-strict mode. Since x is already defined (from line 01), z is set to 3.14. Therefore, statement B is correct: z is equal to 3.14.
Scope of 'use strict' inside a function
The line:
'use strict';
inside myFunction is a directive prologue for that function, not for the entire file. This means:
Strict mode applies only within the body of myFunction, from the directive to the end of that function.
It does not retroactively affect code before the function or code outside of it.
Therefore:
Statement A is incorrect: 'use strict' is not "hoisted" to affect the whole file. It only affects the function body.
Statement C is incorrect: strict mode does not apply from line 04 to the end of the file; it only applies within myFunction, not to global lines like 01, 08, or 09.
Execution of myFunction in strict mode
When myFunction is called on line 09:
function myFunction() {
'use strict';
y = x;
}
Within this function:
Strict mode is active for its body.
In strict mode, assigning to an undeclared variable (like y here) is not allowed and results in a ReferenceError at runtime.
So line 05:
y = x;
throws a ReferenceError because y is not declared with var, let, or const.
Therefore, statement E is correct: line 05 throws an error.
Why statement D is considered correct
Statement D says:
'use strict' has an effect only on line 05.
In terms of execution in this specific code:
The only executable statement in myFunction that is affected by strict mode is the assignment on line 05.
The directive itself on line 04 is not a "normal" runtime operation; it is a directive that sets the mode.
There are no other statements inside the function that behave differently under strict mode; only the line that assigns to the undeclared variable shows a strict-mode effect.
So, describing the practical execution behavior of this snippet, strict mode manifests its effect only on line 05, making statement D correct in this context.
Summary of each option:
A: Incorrect - strict does not apply to all lines in the file.
B: Correct - z becomes 3.14 in non-strict global code.
C: Incorrect - strict does not affect code outside the function.
D: Correct - in this code, the only behavioral effect of strict mode is on line 05.
E: Correct - line 05 throws a ReferenceError due to assignment to undeclared y under strict mode.
JavaScript knowledge references (descriptive, no links):
In non-strict (sloppy) mode, assigning to an undeclared identifier creates a global variable.
'use strict' inside a function body enables strict mode for that function only.
In strict mode, assigning to an undeclared variable results in a ReferenceError.
Directive prologues ('use strict') affect only their containing function or script, not other scopes.
NEW QUESTION # 108
Given two expressions var1 and var2, what are two valid ways to return the concatenation of the two expressions and ensure it is data type string?
Answer: A,D
Explanation:
We want to guarantee that the result is of type string regardless of what var1 and var2 are (numbers, booleans, etc.).
Option A:
String(var1).concat(var2)
String(var1) converts var1 explicitly to a string.
.concat(var2) calls String.prototype.concat, which:
Converts var2 to string if needed.
Returns a new string that is the concatenation of the two.
Result is always a string.
So A is valid.
Option B:
String.concat(var1 + var2)
String.concat (as a static method) does not exist. concat is an instance method on string objects, not on the String constructor.
var1 + var2 might be a number (if both numeric) and used incorrectly here.
So B is invalid.
Option C:
var1 + var2
+ can mean:
String concatenation (if at least one operand is a string).
Numeric addition (if both operands are numbers).
If var1 and var2 are both numbers, var1 + var2 will be a number, not a string.
This does not ensure the result is always a string.
So C is not guaranteed to satisfy the requirement.
Option D:
var1.toString() + var2.toString()
var1.toString() converts var1 to a string (for most primitive/object types).
var2.toString() does the same.
Now both operands of + are strings, so the result is string concatenation.
Result is guaranteed to be a string.
So D is valid.
Therefore, the two valid ways are A and D.
Study Guide / Concept Reference (no links):
String() conversion vs .toString()
String.prototype.concat
+ operator overload: numeric addition vs string concatenation
Type coercion rules for primitives
NEW QUESTION # 109
Refer to the code below:
What is the value of result when the code executes?
Answer: C
NEW QUESTION # 110
Given a value, which three options can a developer use to detect if the value is NaN?
Answer: A,B,D
Explanation:
In JavaScript, NaN has some special properties:
NaN is not equal to anything, including itself.
Equality operators (==, ===) do not work for detecting NaN.
Check each option:
A . value === Number.NaN
Number.NaN is just NaN.
But NaN === NaN is always false.
So this expression is never true and cannot be used to detect NaN.
B . value == NaN
Same issue. NaN == NaN is always false.
So this also never detects NaN.
C . Object.is(value, NaN)
Object.is is a more precise equality comparison method.
It treats NaN as equal to NaN: Object.is(NaN, NaN) is true.
So Object.is(value, NaN) is a valid way to detect NaN.
D . value !== value
Because NaN is the only JavaScript value that is not equal to itself,
value !== value is true only when value is NaN.
This is a classic trick for NaN detection.
E . Number.isNaN(value)
Number.isNaN is the recommended modern way to check if a value is NaN without coercion.
It returns true only if value is actually the number NaN.
Therefore, the three correct ways are:
Object.is(value, NaN)
value !== value
Number.isNaN(value)
NEW QUESTION # 111
Given the code below:
What is logged to the console'
Answer: D
NEW QUESTION # 112
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